The present invention relates generally to a system for the recycle of argon. More specifically, the present invention relates generally to a system for the recovery, purification, and recycle of argon, for example, from high temperature furnaces such as silicon crystal growth furnaces.
Argon may be employed in various processes wherein its chemically inert nature, specific physical properties, and cost, which may be lower relative to other inert gases, makes its use particularly advantageous. Argon may be used as a blanketing gas, a purge gas, or a heat transfer medium. For example, high-purity argon (Ar), having less than about 1 ppb impurities, may be used as the inert gas within a high temperature, silicon crystal growth furnace.
Silicon substrates used in the production of semiconductors are typically manufactured in a two-step process such as the Czochralski method. In the first step or forming step, large single crystals of silicon are grown in a high temperature furnace by heating silicon in an argon atmosphere. A seed of silicon is dipped into molten silicon and slowly withdrawn. As the seed is withdrawn, a large ingot of single-crystal silicon is formed onto the crystal seed. Dopants such as arsenic, phosphorous, or boron may be added to either the molten silicon or to the argon purge stream to add trace levels of impurities to the crystals in order to alter the electronic properties of the crystal. In the second or finishing step, the silicon crystal may be cooled under an argon atmosphere to ambient temperature to form an ingot. The ingot is polished and then sliced into wafers for further processing.
The argon effluent from the high temperature furnace may contain trace levels of volatile dopants, oxygen, nitrogen, carbon oxides (i.e., CO, CO2), hydrogen, and hydrocarbons (i.e., CH4, C2H4, C3H6, etc.). Because the utilization of argon is large and the availability of argon on the commercial market may be modest, various attempts to reutilize the argon have been identified in the prior art. Each of these attempts has tried to overcome the problem of taking the contaminated, impure argon effluent from the furnace and treating it to remove particulates, dopants, and/or other impurities, typically in the form of gases, for recycle of acceptably pure argon for reuse in the silicon crystal furnace. A key criterion for the recovery and purification of such argon is that the process be competitive with the commercial costs of fresh argon from the industrial gas market.
U.S. Pat. No. 5,106,399 describes an argon purification system. The impure argon is first passed through a molecular sieve adsorbent bed to remove water vapor and carbon dioxide. The dehydrated, impure argon is then passed through a bed of catalytic material to chemisorb oxygen, hydrogen, and carbon monoxide. Lastly, the argon is passed through an adsorbent bed at cryogenic temperatures to adsorb nitrogen and hydrocarbons before recovering a purified argon stream for reuse. The '399 patent, however, does not specifically address how the dopant materials are removed from the argon stream. Further, a portion of the purified argon may be lost during the regeneration cycle.
U.S. Pat. No. 5,706,674, assigned to the assignee of the present invention, describes a process for recovery of argon from silicon crystal furnaces. The impure argon is recovered from the furnace and compressed to an elevated pressure. The argon is then passed through a scrubber to remove any dopant compounds contained therein. Next, the impure argon is passed through a deoxygenation unit with hydrogen, if needed, to reduce the oxygen concentration and produce water and CO2. The impure argon is then passed through a molecular sieve drier to remove water and CO2. Lastly, the argon is purified using cryogenic separation, either distillation or adsorption, to remove oxygen, H2, CO, and trace hydrocarbons.
U.S. Pat. No. 6,123,909 describes a process for purifying Ar contaminated with CO, O2, N2, and methane. Air or oxygen is added to the impure argon and the combined stream is passed across a catalyst bed that selectively oxidizes the CO but not the methane contained therein to form CO2. Hydrogen gas is then added to the stream to selectively react with oxygen over a catalyst to form H2O. The argon stream is then passed over an adsorbent to remove the CO2 and H2O by-products of the prior steps. The resulting gas stream is then cooled to cryogenic temperatures and fed to a distillation column to remove the N2, H2 and methane from the argon stream. Like the '399 patent, the '909 patent does not discuss the removal of dopants from the argon gas stream.
U.S. Pat. No. 6,113,869 describes a process to purify an Ar stream contaminated with H2, CO, water vapor, O2, CO2, N2 and methane. The argon stream is first dried using a dessicant to remove water vapor. The stream then contacts an oxidation catalyst in the presence of excess oxygen, if needed, to completely react the CO and H2 to form CO2 and H2O. The resulting stream is passed over an adsorbent to remove the CO2 and water produced in the prior step. The excess oxygen added in the previous step is removed by contacting the stream with a metal catalyst at temperatures greater than approximately 75° C. Lastly, the nitrogen and methane within the argon stream is removed by cryogenic adsorption. Like the '399 and '909 patents, there is no discussion of dopant removal in the '869 patent.
Japanese Published Patent Application No. 4-89387 describes a process for purifying argon from a silicon crystal furnace in which the gas is removed from the furnace using a vacuum pump. Any particles contained within the gas are removed by filtering the pump inlet. The gas is then passed to a pressure swing adsorption bed containing a zeolite molecular sieve to remove carbon monoxide, nitrogen, and carbon dioxide. The resultant gas stream is then passed to a palladium catalyst to convert oxygen to water and then to an additional adsorption system to dry the gas.
Japanese Published Patent Application No. 7-33581 describes a process for purifying argon from a silicon crystal furnace in which the gas is removed from the furnace using a vacuum pump. The resultant gas is passed to a catalyst to convert carbon monoxide, hydrogen and oxygen to carbon dioxide and water. The resultant gas stream is then sent to an adsorption system to remove carbon dioxide and water.
Japanese Published Patent Application No. 6-24962 describes a multi-step process for purifying argon from a crystal-pulling furnace. Impure argon is passed to a venturi scrubber to remove particulates and then passed to a series of catalyst and adsorption beds. The argon gas is then passed to a bed of de-oxo catalyst to remove oxygen. Next, the gas is sent to a bed of cupric oxide catalyst to convert carbon monoxide and hydrogen within the gas to carbon dioxide and water. The water and carbon dioxide are then removed in a zeolite adsorption bed. Lastly, nitrogen is removed in a low-temperature sorption bed.
Accordingly, there is a need in the art to provide a method for the recovery, purification, and recycle of high purity argon from a silicon crystal furnace. There is a need in the art for a process to remove dopants, as well as other impurities, from an argon gas stream. There is also a need in the art for a cost-effective process to minimize the loss of argon within the purification process. Lastly, there is a need in the art for a process that minimizes capital equipment costs such as the costs of procuring and maintaining relatively large cryogenic units or catalyst beds.
All references cited herein are incorporated herein by reference in their entirety.